In nonvolatile semiconductor storage devices currently in the mainstream, as represented by NAND flash memories, the memory cell is constituted by a charge accumulation transistor. In the charge accumulation transistor, a charge accumulation layer is formed in the insulating film between the channel layer and the gate electrode. Accumulation of charge in this charge accumulation layer changes the threshold voltage of the charge accumulation transistor. This is utilized to perform writing and reading of information. In order to increase the capacity in such a nonvolatile semiconductor storage device, further miniaturization of the charge accumulation transistor is required. However, as the insulating film for retaining charge is made thinner, the leakage current is increased, and hence the charge retention capability is degraded. Thus, in a nonvolatile semiconductor storage device using charge accumulation transistors as memory cells, the increase of capacity is becoming difficult.
In this context, a nonvolatile semiconductor storage device with cross-point memory cells arrayed in a memory cell array has been drawing attention. The cross-point memory cell includes a variable resistance element as a memory element at the intersection between a word line and a bit line. In such a nonvolatile semiconductor storage device, the change of the resistance of the memory cell is used as information. Thus, compared with the memory cell of the charge accumulation type, miniaturization causes less degradation in the storage retention operation. Hence, the nonvolatile semiconductor storage device including cross-point memory cells is promising for increasing the capacity. In particular, CBRAM (conductive bridging random access memory) is promising as a cross-point nonvolatile semiconductor storage device. In CBRAM, metal ions are diffused in the diffusion layer sandwiched between the electrodes of the memory cell. Thus, the formation of a current path connecting the electrodes is controlled to change the resistance of the memory cell. Furthermore, in conjunction with the variable resistance characteristics, the memory cell of CBRAM has rectification characteristics as well. In the cross-point nonvolatile semiconductor device, in general, in the read operation, a reverse bias is applied to the non-selected memory cells. Thus, the current flowing into the non-selected memory cell needs to be suppressed. Hence, a rectifying element such as a diode needs to be series connected to the memory cell. However, in CBRAM, the memory cell itself has rectification characteristics. This eliminates the need of additional connection of a rectifying element. Thus, because of the simplified structure of the memory cell, CBRAM is promising for increasing the capacity.
However, if the size of the memory cell is 10 nm or less, it is difficult to control the formation of the current path by diffusion of metal ions. For instance, metal ions may be excessively diffused into the diffusion layer. This increases variations in the switching characteristics for repeating the ON state and the OFF state of the memory cell. Furthermore, the durability in the switching characteristics may be degraded. As the memory cell size becomes smaller, the influence of excessive diffusion of metal ions is greater. With the decrease of the resistance of the memory cell, the rectification characteristics are also degraded, and the reverse current increases. On the other hand, if the ion diffusion layer is made of a material suppressing the diffusion of metal ions, the ion diffusion layer has poorer adhesiveness to the electrode serving as a diffusion source of metal ions. Consequently, in the manufacturing process, voids are generated at the interface between the electrode and the ion diffusion layer. This causes the yield decrease and failures of the cells. There is demand for CBRAM including memory cells with good adhesiveness between the ion diffusion layer and the electrode serving as a diffusion source, good rectification characteristics, and high durability in the switching characteristics.